Averages and Probability

Read this text. Pay close attention to the "How To" sections which give summaries of how to do mean, median, and mode calculations. After you read the text, complete the practice problems and check your answers.

Averages and Probability

One application of decimals that arises often is finding the average of a set of numbers. What do you think of when you hear the word average? Is it your grade point average, the average rent for an apartment in your city, the batting average of a player on your favorite baseball team? The average is a typical value in a set of numerical data. Calculating an average sometimes involves working with decimal numbers. In this section, we will look at three different ways to calculate an average.

Calculate the Mean of a Set of Numbers

The mean is often called the arithmetic average. It is computed by dividing the sum of the values by the number of values. Students want to know the mean of their test scores. Climatologists report that the mean temperature has, or has not, changed. City planners are interested in the mean household size.

Suppose Ethan's first three test scores were $85$ , $88$ , and $94$ . To find the mean score, he would add them and divide by $3$ .

$\frac{85+88+94}{3}$

$\frac{267}{3}$

$89$

His mean test score is $89$ points.

The Mean

The mean of a set of $n$ numbers is the arithmetic average of the numbers.

$\text{mean}=\frac{\text{sum of values in data set}}{n}$

How To

Calculate the mean of a set of numbers.

Step 1. Write the formula for the mean

$\text{mean}=\frac{\text{sum of values in data set}}{n}$

Step 2. Find the sum of all the values in the set. Write the sum in the numerator.
Step 3. Count the number,

$n$ , of values in the set. Write this number in the denominator.
Step 4. Simplify the fraction.
Step 5. Check to see that the mean is reasonable. It should be greater than the least number and less than the greatest number in the set.

Did you notice that in the last example, while all the numbers were whole numbers, the mean was 59.5, a number with one decimal place? It is customary to report the mean to one more decimal place than the original numbers. In the next example, all the numbers represent money, and it will make sense to report the mean in dollars and cents.

Find the Median of a Set of Numbers

When Ann, Bianca, Dora, Eve, and Francine sing together on stage, they line up in order of their heights. Their heights, in inches, are shown in Table 5.6.

Ann	Bianca	Dora	Eve	Francine
59	60	65	68	70

Table 5.6

Dora is in the middle of the group. Her height, 65'', is the median of the girls' heights. Half of the heights are less than or equal to Dora's height, and half are greater than or equal. The median is the middle value.

The numbers 59, 60, 65, 68, and 70 are listed. 59 and 60 have a brace beneath them and in red are labeled "2 below". 68 and 7

Median

The median of a set of data values is the middle value.

Half the data values are less than or equal to the median.
Half the data values are greater than or equal to the median.

What if Carmen, the pianist, joins the singing group on stage? Carmen is 62 inches tall, so she fits in the height order between Bianca and Dora. Now the data set looks like this:

$59, 60, 62, 65, 68, 70$

There is no single middle value. The heights of the six girls can be divided into two equal parts.

Statisticians have agreed that in cases like this the median is the mean of the two values closest to the middle. So the median is the mean of

$62$ and

$65$ ,

$\frac{62+65}{2}$ . The median height is

$63.5$ inches.

The numbers 9, 11, 12, 13, 15, 18, and 19 are listed. 9, 11, and 12 have a brace beneath them and are labeled "3 below". 15,

Notice that when the number of girls was 5, the median was the third height, but when the number of girls was 6, the median was the mean of the third and fourth heights. In general, when the number of values is odd, the median will be the one value in the middle, but when the number is even, the median is the mean of the two middle values.

How To

Find the median of a set of numbers.

Step 1. List the numbers from smallest to largest.
Step 2. Count how many numbers are in the set. Call this

$n$ .

Step 3. Is

$n$ odd or even?

$n$ is an odd number, the median is the middle value.

$n$ is an even number, the median is the mean of the two middle values.

Identify the Mode of a Set of Numbers

The average is one number in a set of numbers that is somehow typical of the whole set of numbers. The mean and median are both often called the average. Yes, it can be confusing when the word average refers to two different numbers, the mean and the median! In fact, there is a third number that is also an average. This average is the mode. The mode of a set of numbers is the number that occurs the most. The frequency, is the number of times a number occurs. So the mode of a set of numbers is the number with the highest frequency.

Mode

The mode of a set of numbers is the number with the highest frequency.

Suppose Jolene kept track of the number of miles she ran since the start of the month, as shown in Figure 5.7.

An image of a calendar is shown. On Thursday the first, labeled New Year's Day, is written 2 mi. On Saturday the third is wri

Figure 5.7

If we list the numbers in order it is easier to identify the one with the highest frequency.

$2, 3, 5, 8, 8, 8, 15$

Jolene ran 8 miles three times, and every other distance is listed only once. So the mode of the data is 8 miles.

How To

Identify the mode of a set of numbers.

Step 1. List the data values in numerical order.
Step 2. Count the number of times each value appears.
Step 3. The mode is the value with the highest frequency.

Some data sets do not have a mode because no value appears more than any other. And some data sets have more than one mode. In a given set, if two or more data values have the same highest frequency, we say they are all modes.

Source: Rice University, https://openstax.org/books/prealgebra/pages/5-5-averages-and-probability
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